The invention relates to an exhaust muffler for an internal combustion engine, especially of a portable implement, comprising a catalyst which is arranged in a housing, which has at least one inlet opening and one outlet opening for an exhaust gas flowing from an internal combustion engine into an environment.
The exhaust gases from internal combustion engines generally enter the catalyst at an exhaust gas temperature of about 600° C. A chemical conversion of the exhaust gases takes place inside the catalyst. In this case, three processes take place simultaneously adjacent to one another: NOx is reduced to nitrogen, releasing oxygen, CO is oxidized to CO2 and HC compounds are oxidized to CO2 and H2O, consuming oxygen at the same time. As a result of the chemical conversion processes, the temperature of exhaust gas “converted” by the catalyst is increased to about 1000° C. When this heated exhaust gas emerges from the catalyst, there is a risk of afterburning if sufficient HC and O2 is present and the ignition temperature is exceeded.
This is especially disadvantageous in the case of portable implements since the operator of such an implement can be injured by flame formation or combustible materials located in the vicinity can be ignited.
Described from DE 79 25 614 U1 is an exhaust muffler which has an inlet space and an outlet space from which an exhaust gas is led away. In the area of the outlet a branched partial stream of cooling air is supplied to the exhaust gas stream, which is mixed with the exhaust gas at the outlet and provides for a reduction in temperature. It is disadvantageous however that a sufficient reduction in the temperature of the exhaust gas stream is not achieved in the area of the outlet.
Known in accordance with DE 1 98 34 822 A1 is an exhaust muffler which is mounted on an internal combustion engine of a hand-held implement. In this case, the exhaust muffler has a housing with a catalyst element which is positioned between an exhaust gas inlet and an exhaust gas outlet.
Furthermore, a first partial stream of the exhaust gas stream inflowing via the exhaust gas inlet is brought in contact with the catalyst element and a second partial stream flows substantially without contact with the catalyst element to the exhaust gas outlet. Before leaving the housing, the partial streams are brought together and mixed. In this case also no satisfactory reduction in the exhaust gas outlet temperature is achieved, whereby the afore-mentioned hazards could be avoided.
DE 38 29 668 C3 proposes an exhaust gas muffler in which a catalyst is arranged with a spacing on all sides [inside] a housing of the exhaust gas muffler, wherein the converted exhaust gas is led out of the catalyst to the outlet through an exhaust gas end pipe, wherein the exhaust gas end pipe lies inside the muffler housing over most of its length with spacing on all sides and is flushed with untreated exhaust gas, and the treated exhaust gas is led away from the muffler housing into the ambient air through a starting section of the exhaust gas end pipe. This cooling of the pipe behind the catalyst inside the muffler by unconverted exhaust gas is inferior to the additional cooling by ambient air and the muffler is expensive and costly to produce since the pipe must be made of stainless steel for example and there is a high degree of forming. In addition, the proposed solution merely leads to short pipe lengths since the length must be realized inside the muffler.